Evaporating method and means therefor



Feb. 25, 1936. E. OMAN EVAFORATING METHOD AND MEANS THEREFOR Original Filed May 14, 1929 2 Sheets-Sheet l 80 90 grees G de 275/779 Air i m'm Feb. 25, 1936. E. OMAN 2,032,132

EVAPORATING METHOD AND MEANS THEREFOR Original Filed May 14, 1929 2 Sheets-Sheet 2 Patented Feb. 25, .1936

UNITED STATES PATENT OFFICE EVAPORATING METHOD AND MEAN THEREFOR Sweden Substitute for application Serial No. 362,976, May 14, 1929. This application April 26, 1934, Serial No. 722,591. In Sweden May 18, 1928 Claims. (CL 159-16) This invention relates to methods and apparatus for evaporating liquids, and more particularly to the evaporation of water or other liquids from solutions by the utilization of heat that would normally be wasted.

In a great number of industries it is of great economical importance to be able to evaporate liquids, especially water, with the expenditure-of as small a quantity of heat as possible. To this end, some evaporating systems have made use of apparatus of the multiple efiect type, while other systems have used a so-called heat pump for compressing exhaust steam which is then utilized as the heating medium for evaporating purposes.

It is one of the objects of the present invention to provide a novel method and apparatus for evaporating liquids wherein the evaporation is accomplishedin aplurality of stages each of which operates under substantially the same pressure both inside and outside of the apparatus and which utilize as the heating medium exhaust steam, flue gases and like sources of what is normally considered as waste heat.

Another object is to provide a method and apparatus of improved character for evaporating liquids wherein heat is transferred from a condensible gas-vapor mixture through a partition to a layer of the liquid or solution to be evaporated flowing on the opposite side thereof, while at the same time heat is transferred by evaporation from the liquid or solution to a heat absorbing medium including a non-condensible gas which is flowing in direct contact with, and preferably countercurrent to, the surface of the liquid layer.

A further object is to provide a novel evaporating system of this geneal character wherein the gasvapor mixture constituting the heat absorbing medium in one stage is utilized as the heat yielding medium in another stage, and is even susceptible of subsequent repeated usage as the heatabsorbing and heat yielding medium, alternatively,

These and other objects will appear more fully from a consideration of the detailed description of the invention that follows. Although a number of difiercnt mechanical embodiments of apparatus for carrying out the present invention are described and illustrated in the accompanying drawings, it is to be expressly understood that these drawings are for purposes of illustration only and are not to be construed as defining the scope of the invention, reference being had to the-appended claims for the latter purpose.

In the drawings:

Fig. 1 is a diagrammatic illustration of one form of apparatus embodying the structural features, and capable of carrying out the method, of the present invention;

Figs. 2a and 2b are diagrams illustrative of the character and emciency of the heat transmission from the condenslble gas-vapor mixture to the liquid to be evaporated;

Fig. 3 is another diagrammatic illustration of a multi-stage evaporating system embodying the present invention; 1

Figs. 4, 5, 6, '7 and 10 illustrate various other forms of apparatus embodying the invention; and

Figs. 8a., 8b, 9a, 9b and 9c illustrate various means which may be employed for efiecting thorough mixing of the heat absorbing medium and well-spread distribution of the flowing liquid to be evaporated.

The principle of the present invention is illustrated diagrammatically in Fig. 1, wherein there is disclosed an evaporating system of two stages employing heat exchangers A and B, each of which comprises a heating chamber and an evaporating chamber separated by a heat transmitting wall. In the first stage or exchanger A, a heating medium comprising a mixture of a permanent gas, such as air, and a condensible vapor, preferably at atmospheric pressure, is introduced at a into the heating chamber b and flows therethrough in heat yielding relation with the wall thereof, the condensate and uncondensed portion of the gas-vapor mixture escaping at c. From the chamber b, heat is transferred into the liquid or solution to be evaporated which is contained in the vessel 11 constituting an evaporating chamber. Air or some other suitable permanent gas, preferably at atmospheric pressure, is then conducted over the surface of the solution in evaporating chamberd, said gas being introduced The air escaping from the first exchanger A,

which is saturated or approximately saturated with the vapor of the evaporated liquid, is then introduced through the pipe I into the heating chamber 9 of the second stage or exchanger B and is there utilized as the heating medium, a portion of its vapor being condensed and giving up its heat to the solution in the vessel or evapcrating chamber It, said solution having, of course, a lower temperature than the air-vapor mixture present in heating chamber g. Condensate and air saturated with vapor escape from heating chamber a at l, the temperature of the air-vapor mixture being lower at 1 than it is at f. Air also introduced to evaporating chamber h at i, this air being conducted over the surface of the solution therein so as to cause vapor to be absorbed by the air in a manner similar to the evaporation in the first exchanger A. Air and absorbed vapor escape from evaporating chamber 7!. at k. and, iidesired, this air-vapor mixture may be used as the heating medium in the heating chamber of a third exchanger, and soon. The air introduced at i must have such a temperature and such a degree oi. saturation, and must be introduced in such quantity, that the temperature of the solution in evaporating chamber It will always be lower than that of the heat yielding air-vapor mixture flowing through heating chamber 9.

The solution to be concentrated by evaporation is supplied at certain temperatures to evaporating chambers d and h, respectively, and is conductedtherei'rom through any suitable means such that a suitable liquid level and a desired concentration are maintained and obtained. The method and apparatus described is obviously applicable to any solution .and any solvent. Likewise, any suitable gas or gas mixture, such as nitrogen, carbon dioxide. etc... may be introduced at e and i as the heat absorbing medium instead of air. It is also possible to employ either horizontal, verticai or inclined heat transmitting walls between the heating and evaporating chambers.

Decisive of the technical importance of the method of the present invention is the high de= gree of heat transmission obtained from the air= vapor mixture at condensation and through a metal wall to the streaming liquid solution to be evaporated. This is illustrated in Figs. 2a and 2b. The drop in temperature from vapor saturated air aa, through awall be to a flowing liquid cs is shown diagrammaticallg in Fig. 2a.. The quantity of heat which is transferred in such a system for each square meter of wall surface during a period of one hour and at a temperature differenceof 1 C.-between as and C6 has been determined experimentally for water, and the values of the coeflicient of heat transmission so obtained are shown as ordinates on the diagram of Fig. 2b, the abscissee of said diagram representing the temperature in degrees centigrade of the condensible air-vapor mixture. Thus, the diagram 01' Fig. 2b rep-resents the coefficient of heat transmission measured in kg. cal/m, 1, hour as a function of the temperature of the condensible air-vapor mixture. be displaced somewhat with changes in the velocities or the air-vapor mixture and the water, but the values in Fig. 2b refer to normal velocities of the order at 5 meters'per second. It is apparent from the curve in Fig. 21) that this ceeflicient of heat transmission is very great.

As has previously been stated, in evaporating systems of the character described the air er gas constituting the heat absorbing medium flowing through the evaporating chamber must he of such a temperature and such a degree 01 sata- This curve will obviously can take place only when the tempereture of the liquid solution is at every point lower than the temperature of the gas-vapor mixture passing on the opposite side of the heat transmitting wall dividing the heating and evaporating chambers, since the gas-vapor mixture must be continually undergoing condensation. However, at the same time that the liquid solution is abserbing heat indirectly from the condensible gas-vapor heating medium, its temperature is also being lowered by evaporation from its surface over which is being flowed a current oi unsaturated air or gas which, by absorption of heat and vapor from the liqeid solution, gradually approaches saturation.

In order that evaporation accompanied by simultaneous cooling or the lieuid layer may occur at each point on the surface or the liquid solution, it is necessary that the total heat con-. tent of the heat absorbing air or gas mixture ly, since the condition for existence of a heat transfer from the heat yielding medium to the liquid solution at the same point of the wall is that the temperature of the solution be lower than that of the heat yielding medium, it will be evident that, in order for the method oi. the present invention to be most emciently carried out, the total heat content of the heat absorbing medium must be lower than that of the heat yielrhng medium. In other words, unless at every point the total heat content of the condensible gas-vapor heat yielding medium is greater than the total heat content oithe heat absorbing air or gas plus its contained vapor, evaporation without boiling and accompanied by cooling of the liquid solution being evaporated will not-be effectively obtained.

Fig. 3 illustrates diagrammatically a method and apparatus for effecting evaporation according to the present invention in four stages or heat exchange units connected in series, it being assumed that air is the permanent gas which circulates in the system. The four stages or heat exchange units are designated by I, II, III and IV, the temperatures being lowest in unit I and highest in unit IV. The solution to be evapopassing over the solution be lower than the total rated is injected int-o evaporating chambers c1,

ca, ca and 04 at a1, a2, a; and m, respectively, having been preheated to different temperatures, the lowest temperature prevailing at m and the highest at at. The solution is injected upon the heating surfaces in any suitable manner such that the latter are entirely covered by a layer of flowing liquid, the unevaporated portion or which,

upon reaching the bottoms of the units in qnestion, is conducted 01! at b1. b2, b3 and b4, re-

rated air of relatively low temperature is sucked through the channel H into the evaporating chamber or of heat exchange unit I, whereinthe air will flow upwardly countercurrent to the solution and absorb vapor therefrom, the degree of saturation of the air thereby'being progressively increased accompanied by a transfer of heat from and a corresponding progressive decrease in temperature of the solution. From the top of chamber oi. the mixture of air and its absorbed vapor is conducted through the pipe (11 to the evaporating chamber 0: of unit II wherein it is conducted over the surface of a flowing solution of a temperature higher than that of the solution in.

chamber 01. Therefore, in chamber or the saturation temperature of the air will be increased and it will absorb a further quantity of vapor. Leaving chamber ca the air is conducted through the pipe dz to the evaporating chamber 63 of unit III, wherein the temperature is higher than in on and again results in additional absorption of vapor by the air from the solution, and thence through the pipe (is to the evaporating chamber 04 of unit IV where a still higher temperature prevails and a still further quantity of vapor is absorbed.

After leaving the unit IV, the mixture of air and its absorbed vapor is conducted through the pipe at; to a heating apparatus 01 wherein it may be heated by means of a suitable steam coil M and supplied with additional water or solution injected through a pipe B1. Thus the air will be completely saturated with. vapor, and the airvapor mixture raised-to a higher temperature than before. It is also possible to introduce steam directly from. any suitable steam generator (not shown) into the air-vapor mixture in apparatus C1. The air should, however, when leaving apparatus C1, be completely saturated and of a temperature which is higher than that of the liquid solution which is supplied to evaporating chamber 04.

The hot saturated air is conducted from the apparatus C1 through pipe m to the heating chamber :24 of stage or unit IV. By reason of the fact that the air-vapor mixture entering the chamber e; is of a higher temperature than the liquid entering at a4, condensation of some of the vapor takes place in the chamber er and the heat of, condensation is transferred through the wall of said chamber to the flowing liquid solution in evaporating chamber or, which quantity of heat is in turn consumed in the formation of vapor and its absorption by the air in 04. The water of condensation obtained in the chamber e4 is collected at the bottom thereof and discharged therefrom at f4. From the chamber er the condensibie air-vapor mixture is conducted through the pipe as to the heating chamber es of unit III. Inasmuch as the temperature of the solution entering at as is lower than that of the solution entering at at, the air-vapor mixture supplied to the chamber e3 will have a higher temperature than the solution in the chamber 03, a-condition which causes heat to be transferred from ea to on, evaporation to occur in C3, and condensation to occur in es, the water of condensation beingcollected at the bottom and conducted of! through pipe is. Similarly, the temperature in the heating chamber 62 will be higher than in the evaporating chamber or, and higher in chamber e1 than in chamber m, which results in heat being transferred from e: to c: and from (21 to or, additional evaporation in c: and c1, and additional condensation in e: and er. The condensate from e: is conducted off at 12, while the condensate from the chamber ei is discharged at f1.

. The airand any remaining uncondensed vapor leaving the chamber-e1 is conducted through the pipe gt to the condenser D, wherein the air is cooled by direct or indirect cooling. The condensate, or with direct cooling, both the condensate and the cooling water, escapes through the liquid-sealed pipe E. The cooled and relatively dry air is then conducted through H back to the evaporating chamber 01 of unit I, and again circulated through the evaporating chambers c1, ca, ca and c4 and through the heating apparatus 01 by the fan A1, and then back through the heating chambers e4, es, e: and e1, and so on. The heating in apparatus C1 and the subsequent cooling in chamber D are useful steps in the prevention of heat stabilization within the system.

Through the liquid seal F fresh air is sucked into the system automatically in case the pressure in the apparatus should sink below atmospheric, while through the liquid seal E air is automatically blown out in case the pressure should rise above atmospheric. These two liquid seals may be made adjustable, whereby the pressure in the apparatus may be kept approxipresent system operates at substantially atmospheric pressure.

Instead of water, a solution may be added through pipe B1 in heating apparatus C1, and concentration of this solution then obtained by vaporization in C1 due to the heating effect of coil M. The unevaporated water or concentrated solution will flow out of the heating apparatus C1 through liquid seal 0.

By operating the method and apparatus of the present invention at substantially the same pressure interiorly and exteriorly, the problems hitherto presented to the art of preventing leakage, providing heavy or strong construction to resist pressure differences, providing vacuum pumps of sufficient capacity, etc., are avoided.

The method of the invention is also such as to assure against operating conditions that are likely to produce precipitation or incrustation on the heat transmitting wall either from the liquid being concentrated or from the gas used as the heating medium, if they should contain ingredients likely to form scale. In the present method, evaporation is effected without the production of boiling within the liquid being concentrated, and there is therefore no danger that the flowing layer of liquid will become so thin, or its concentration so great, because of localized heat conditions, that precipitation or incrustation will ensue. Since the system operates at atmospheric pressure and without boiling, the

temperature within the apparatus is always less than 100 C. Actually, under preferred operating conditions, this temperature seldom exceeds a maximum of 85 C. Since the highest temperature obviously prevails at the point from which the heat is initially transferred, that is to say in the condensible gas-vapor mixture, it is obvious that the liquid solution which is evaporating will not be subjected to a temperature even as high as 85 0., the result being that incrusta-,

tion on the heat transmitting surfaces during treatment of those liquids which tend to form scale is reduced to a These incrustations will not only occur on the solution side of the heat transmitting wall but also on the heating side, provided that the heating medium is such as to possess a scale forming tendency. For example, if flue-gases are used as the heating medium, it is necessary to keep the temperature of the flue-gases as low as possible so that the incrusting constituents will not be permitted to precipitate. The solubility of such constituents decreases with an increase of temperature, which fact is also conducive to the use of a condensible heat yielding medium. Not only are proper temperatures thus maintained in the heat yielding medium so as to avoid precipitation and incrustation, but the formation and presence of the condensate also tends to wash the walls and remove any scale that might tend to form thereon.

Moreover, since in the present method vapor is generated from the surface of the liquid solution due to the direct contact therewith of the permanent gas flowing thereover, no boiling occurs within the solution and an increase in concentration of the solution along the heat transmitting wall is avoided, thereby obviating the occurrence of precipitationsat that point. Since the increase in concentration occurs at the free surface of the solution, any precipitations that are formed will not stick to the wall but will be washed away by the unevaporated portion of the solution. This result is further assured by permitting the solution to flow over the heat transmitting surfaces in a sufliciently thick layer.

' Since large quantities of solution flow through the apparatus, the difference in concentration between incoming and outgoing liquid will be small. Consequently, in order to obtain a desired final concentration, the liquid solution must be circulated in the apparatus, as above described. Circulation also promotes a uniform temperature distribution throughout the mass of the solution, so that local overheating thereof is avoided as far as possible. This is of special importance in connection with the evaporation of those solutions which contain substances that are sensitive to certain temperatures, such as wort solutions containing enzymes, ferments or the like, which as a rule do not withstand temthe obtaining of a gradual concentration. For

example, in concentrating a lye solution, circulation of the solution is essential for the purpose of covering the heat transmitting wall with a layer of lye sufliciently thick to assure a substantially uniform heating thereof accompanied by a uniformcooling of the heat yielding medium. If the quantity of lye solution supplied is just enough to obtain the desired final concentration, it may happen, having regard to a practical size of apparatus, that this quantity will be too small to completely cover the heat transmitting wall, with the result that incrustation an'd uneconomical heat utilization will occur. n the other hand, if a suflicient quantity of lye solution is supplied, over-heating at any particular place will be avoided and substantially all of the available heat of the heat yielding medium will be absorbed. Since this will slow up the process of concentration to some extent, circulation of the lye solution isnecessary.

In order to promote even temperature distribution throughout the liquid solution as far as possible during circulation, the heat transmitting crating chambers c are arranged to operate in parallel, that is, each chamber containing solu-' tion and air at the same temperatures as prevail in the other chambers. As shown, the evaporating chambers cand the heating chambers e are disposed alternately, all of the evaporating and heating chambers being preferably connected with common channels for the supply and escape of air, air-vapor mixture, solution and condensate, respectively. a

If it is desired to maintain the vapor absorbing air as saturated as possible at the particular temperatures used, it is preferable that the air be caused to flow along a relatively long path over the surface of the solution. This result, which is particularly desirable in the stage of the system having the highest temperature, may be attained without increasing the height of the apparatus by causing the vapor absorbing air to flow through two or more evaporating chambers in series, which chambers, however, are supplied with solution of the same temperature and have their heating chambers operating in parallel. One embodiment of such an apparatus is illustrated in Fig. 5 wherein the heat yielding airvapor mixture enters both heating chambers e at the same temperature and flows therethrough in parallel, whereas the air intended to absorb vapor from the solution flows first through evaporating chamber c1 and then through chamber 0:.

If it is desired that the heat yielding air-vapor mixture be cooled down as far as possible, it is feasible to connect two or more heating chambers in series, the solution, or the'evaporating air, or both, then being supplied at the same temperature (in parallel) to the corresponding evaporating chambers. I

In Fig. 6 there is illustrated an evaporating system of two stages wherein both the heating chambers and theevaporating chambers are con- .nected in series, but wherein the heat yielding air-vapor mixture in one heating chamber e: flows in the opposite direction, relative to the flow of solution, to that in which it flows in the second'chamber er, the vapor absorbing air likewise flowing oppositely, relative to the flow of solution, in evaporating chamber or to the direction in which it flows in chamber 02. .However,

ner that the stage or unit of highest temperature is heated with the aid of this steam. This is illustrated in Fig. 7 wherein steam is introduced into heating chamber e4 through s, the water of condensation being drawn off at ii. The vapor absorbing air escaping from the evaporating chamber 04 may then be admixed with a suitable quantity of steam supplied through .91, and, if-

system in the manner previously described. If

desired, the gases leaving the heating chambers, or part of them, after having been cooled by direct or indirect cooling, may be returned through the evaporating chambers to serve as the vapor absorbing medium. It will be understood, however, that where these gases are used for heating purposes only, fresh air or some other suitable vapor absorbing medium will be used for the evaporation. The gases or the air escaping from the last evaporating chamber may, upon saturation, be used again as the heating medium in the same or some other evaporating system so long as said gases or air are introduced into an apparatus operating at a suitable temperature with respect to the temperature of the gas or air, as previously explained.

Neither the vapor absorbing air nor the airvapor mixture used for heating need necessarily flow through all of the units of the apparatus, but may in part be permitted to flow around one or more units, particularly when it is desired to regulate the temperature of the apparatus. Likewise, fresh air, hot air, or the circulating air may. if desired, on having been cooled for condensation, be introduced into any stage of the appa ratus. It is also obvious that air may be drawn of! from the apparatus at any point, if desired. For instance, hot gases, such as flue-gases, may be introduced into the unit of lowest temperature, part of which gases are then removed from the system before reaching the next unit.

In order to eflfect thorough mixing of the vapor absorbing air, the evaporating chambers may be provided with spaced rods 1' or baiiie plates p, as shown in Figs. 8a and 8b, respectively, extending into the path of flow of the air so as to cause eddy currents therein. To eflfect uniform distribution of the flowing liquid or solution, so that the entire heat transmitting surfaces will be covered thereby, the walls or, plates separating the evaporating and heating chambers may be provided with wires in, wire netting, rods or the like, as indicated in Fig. 9a, or ridges or grooves may be pressed into said walls or plates, as shown in Figs. 9b and 90, said ridges or grooves preferably being disposed horizontally.

The heat content of the condensate drawn 01! from the various units of the system may preferably be utilized by conducting the condensate through the various other units, from a higher to a lower temperature, and by introducing the condensate into the heating chambers of said units in such a manner that it is further cooled with a consequent giving of! of heat.

The method of the present invention may also be employed in connection with evaporating devices of the type employing rotating heating surplayed for the concentration of lye solutions. As shown, lye solution is circulated through vessels c1 and or each of which is positioned directly beneath a rotatable D1 and D1. The lowest portions of the peripheries of drums D: and DI dip into the lye solution as it flows through vessels c1 and c: and carry some of the solution around with them during the rotation in the form of a thin film spread on the surface ofthe drums. Vapor absorbing air or gas is drawn in through ds by fan A1 and flows around the drum D1 in the space between the periphery thereof and the outer casing which constitutes the evaporating chamber. During the flow of the vapor absorbing air or gas around the drum, during which time the drum is also rotated, preferably in a direction opposite to the direction of flow of the vapor absorbing medium, heat is absorbed and evaporation takes place from the film of lye solution carried by the drum. The

vapor absorbing medium is then conducted through d1 to the evaporating chamber surrounding drum Ds wherein additional vapor is absorbed from the lye solution carried by drum Ds, said solution being at a higher temperature than the solution in vessel 0: with which drum D: is in contact.

The vapor absorbing air or gas is then led through ds to heating chamber 01 wherein its temperature is increased, as by heating coil M, and it is completely saturated at its increased temperature by a spray of water or solution injected into chamber C1 through B1, as previously described. The saturated, highly heated airor gas-vapor mixture is then pumped by fan A1 through as into drum Dc, the interior of said drum constituting a heating chamber ea. In flowing through heating chamber es, the airor gas-vapor mixture acts as a heat yielding medium, some of the vapor being condensed and the heat of condensation being transmitted through drum Do to the lye solution film carried thereby, this heat in turn being transmitted by evaporation to the heat absorbing air or gas flowing around the drum. After passing through heating chamber ea, the heat yielding medium is conducted through mto the heating chamber or within drum D1 wherein still more heat is given up to the solution carried by said drum. Upon leaving heating chamber e1 through go, the heat yielding medium may, if desired, be supplied to still another stage or unit of lower temperature, or it may be cooled and dried and again circulated through the system as the vapor absorbing medium, as previously described in connection with Fig. 3.

There is thus provided by the present invention a new and improved system for effecting evaporation from liquids or solutions which requires only structurally simple and inexpensive apparatus and operates at a relatively high efliciency. The operating pressures in all stages of the system are substantially equal, and are preferably substantially the same as the atmospheric pressure, a feature which obviates many of the difliculties hitherto experienced with evaporating systems requiring pressures both above and below atmospheric. The process of the present invention also makes possible the utilization,- as a source of heat, of flue-gases and other'mediums hitherto regarded as unsatisfactory for heating purposes. The present invention also provides for the eflicient utilization of latent heat and produces increased heat transmission due to the reduction to a minimum of various modifications in the steps of the method have been ldescribed and diagrammatically illustrated in the accompanying drawinga'it will be obvious that the apparatus of the invention is not limited to the forms shown in the drawings, but is capable of a variety of mechanical embodiments, andthat the inventive concept of the method is likewise susceptible'oi embodiment in processes which vary in their details from those disclosed. Beference is therefore to be had to theappended claims for a definition of the limits of the intventionfi This applicatiom is a substitute for my prior application, Serial No. 362,976, filed May 14, 1929. E Z

What is glaimed is:

1. A method of evaporating liquids under substantially the same pressure inside and outside of the apparatus, which includes indirectly transferring heat through an intervening partition to the liquidjto be evaporated by conducting a heat yielding medium, comprising azzmixtuire of gas which'is non-condensing under the conditions of operation and a condensible gapor, along the opposite side of the partition from the liquid to be evaporated tooprogressively increase the temperature of said liquid progressively decrease the temperature of said gas-vapor mixture, andfipassing a heat absorbing medium including a non-condensible gas in direct. contact with the liquid to vaporize said liquid and form a gas-vapor mixture, while maintaining the relative temperaturesaof saidheat yielding medium and said heat absorbing medium and said liquid such that at all points of contact between said liquid andg said heat absorbing medium the total heat content of said heat absorbing medium is less than the total heat content which would exist if the gasvapor mixture constituting said medium were saturated at the temperature of the liquid at said point.

2. A method according to claim 1 wherein the heating and evaporating steps are carried out simultaneously by flowing the heat yielding and heat absorbing media on opposite sides of the intervening partition and wherein the relative temperatures and degrees of saturation of the heat yielding medium and the heat absorbing medium are maintained such that at eaclr: point within the apparatus the total heat content of said heat absorbing medium is less than the total heat content of the heat yielding medium. 1

3. A method according to claim 1 wherein the evaporation is carried on at substantially atmospheric pressure. e

4. A method according to claim 1 which includes the step of recirculating the liquid to be evaporated with respect to said partition.

5. A method according to clain'fi 1 wherein the condensible vapor forming a part of the heat yielding medium is the vapor corresponding to the liquid to be evaporated.

6. A method according to claim 1 which includes the step of conducting the gas-vapor mixture constituting the heat absorbing medium at one stage as the heat yielding medium in heat yieldingrelation with the partition of a second stage. i

7. A method according to claim 1 which includes the steps of conducting the gas vapor mixture constituting the heat absorbing medium "atone stage as the heat yielding medium in heat yielding relation with the partition of a second stage and heating the gas -vapor mixture between its use as the heat absorbing medium and as the heat yielding medium. E r

8. A method according to claim cludes the steps of conducting the gas-vapor mixture constitutingthe heat absorbing medium at one stage as the heat yielding mediunrin heat yielding relation with the partition of a second stage and adding vapor to the gas-vapor mixture r 6 1 which inbetween its use as the heat absorbing mediumi and the heat yielding medium. 7

9. A method according to claim 1 which includes the steps of conducting the gas-vapor g mixture constituting the heat yielding medium 'iieating'and evaporating steps are carried out simultaneously by flowing the heat yielding and heat absorbing media on opposite sides of the intervening partition and wherein the direction of flow of the heat yielding mediim on one side oi. partition is opposite to the direction of flow of the heat absorbing medium on the opposite side of said partition. 1

11. An apparatus for evaporating liquids under substantially the same interior and exterior pressures including a' plurality of heating cham-- bers connected in parallel with one another, a corresponding number of evaporating chambers connected in series with one another, a heat transmitting wall in each of said heatingchambers, means circulating the liquid to be evaporated over eachzof said heat transmitting walls, means circulating a heat yielding medium, comprisinga mixture of gas which is non-condensing under the conditions or operation and a conden sible vapor, through each of said heating chambers along the oppositeside of said heat trans-i mitting wall from said liquid and throughout the "length of said wall to progressively increase the 1 temperature of said liquid and progressively decrease the temperature of said heat yielding medium, and means for conducting a heat absorbing medium including a non condensible gas through ?said evaporating chambers in series andiiver the surface of the liquid therein 7 to progressively 'evaporme liquid from the free surface of said "liquid. J

12. An apparatus for evaporating liquids un-- der substantially the same interior and exterior pressures including a plurality or heating cham bers, a plurality of evaporating chambers, a heat transmitting wall in ieach heating chamber, means circulating the liquid to. be evaporated over each heat transmitting wall, means circu lating a heat yielding medium, comprising a mixture of gas which is min-condensing under the conditions of operation and a condensible vapor, through each heating chamber along the opposite side of thefheat transmitting wall from said liquid and throughout the length of said wall to progressively increase the temperature of said .liquid and progressively decrease the temperature 0; said heat yielding medium, means for conducting a heat absorbing medium including 5 non-condensible gas through said evaporating chambers over the surface of said liquidto pro; gressively evaporate liquid from the free surface of said, liquid, means for withdrawing' the heat absorbing medium with its contained vapors from said plurality of evaporating chambers and adding heat thereto and then returning the same as the heat yielding medium flowing through said heating chambers, and means for withdrawing said gas-vapor mixture from said plurality of heating chambers and withdrawing heat therefrom and then returning the same as the heat absorbing medium flowing through said evaporating chambers.

13. A method ofevaporating liquids under substantially the same pressure inside and outside of the apparatus which includes indirectly transferring heat through an intervening partition to the liquid to be evaporated, said heat substantially consisting of evaporating heat of a heat yielding medium comprising a mixture 01 gas whichris non-condensible under the conditions of operation and a condensing vapor, by conducting said heat yielding medium along the opposite 7 side of the partition from the liquid to be evaporated to progressively increase the temperature I .0! that liquid and progressively decrease the temperature of said gas vapor mixture, and conducting a heat absorbing medium including a non-condensible gas over the surface of the liquid and in a direction opposite to that in which the heat yielding medium flows relative to the liquid to vaporize said liquid from the tree surface thereof and form a gas vapor mixture, while maintaining the relative temperatures of said heat yielding medium and said heat absorbing medium and said liquid such that at all points of contact between said liquid and said heat absorbing medium the total heat content of said heat absorbing medium is less than the total heat content which would exist if said gas vapor mixture were saturated at the temperature of the liquid at said point, and recirculating the liquidvto be evaporated with respect to said partition.

14. A method according to claim 13 which includes the step of conducting the heat yielding medium at one stage as the heat absorbing medium in direct contact with the liquid in a second stage.

15. A method according to claim 13, which includes the step of conducting the heat yielding medium at one stage as the heat absorbing medium in direct contact with the liquid in a second stage and cooling the heat yielding medium before its use as the heat absorbing medium.

ERIK ()MAN. 

